A normal phone device installed in office is connected to four-wire line arrangement including audio signal wires led from an NTT (Nippon Telegram and Telephone Corporation) circuit and wires for transferring control signals between an exchanger and a terminal phone. The four-wire line arrangement has a normalized connector. In actual, however, each manufacturing company has made the corresponding connector providing each arrangement of pin locations, because each manufacturing company may specify a signal for each pin of the connector at its own discretion.
Hence, when any terminal including a phone, a facsimile, and a cordless phone is connected to an already-installed exchanger, an operator cannot grasp which one of the four lines corresponds to audio signals or control signals at a glance and is required to do troublesome operation for specifying it.
In order to overcome the shortcoming, there is provided a phone circuit switching device disclosed in Japanese Patent Application No. Hei 1-42596 (42596/1989), for example. The phone circuit switching device is arranged to use a 8-circuit and 6-contact switch and to switchably connect four wires on the exchanger side to four wires on the phone side through the switch.
This phone circuit switching device is designed to select two wires for audio signals as searching the four wires. Hence, the switching device provides a terminal for picking up an audio signal so that an auxiliary terminal such as a facsimile is switchably connected to the switching device, resulting in allowing a single line to be used for two or more uses.
In case of connecting the auxiliary terminal, when the auxiliary terminal such as a facsimile is stopped, it is desirous to automatically switch back from the facsimile to the phone. The arts disclosed in the Japanese Patent Application Nos. Hei 1-299289 and Hei 1-336916 have been proposed for automatic switching-back operation.
Those automatic switching-back devices are arranged to operate with d.c. power supplied from the control line as a power supply. The energy-saving type exchangers are commercially available so that only alternate current is flown through a control wire without flowing direct current.
Such an automatic switching-back device cannot be applied to those energy-saving type exchangers.